Field devices are used in industrial processes to monitor process variables of the process, or control such process variables. Examples of process variables include pressure, temperature, flow rate, level, etc. Typically, the field devices are located in the “field”, away from a central location. For example, such field devices may be scattered throughout an oil refinery or the like. Field devices are used to transmit the measured process variables back to the control room while controllers are used to control the process from the control room.
Typically, communication between the control room and the field device occurs over a two wire process control loop. Example control loops include 4-20 mA process control loops in which a current level through the loop is used to represent a process variable, control loops operating in accordance with the HART® communications protocol, FoundationFieldus protocol, the Profibus protocol, etc. In many configurations, the field device is also powered with electrical power received over the two wire process control loop or over connectors of the field device via which the field device can be coupled to a power source.
Although field devices typically communicate using process control loops, there are situations in which wireless communications is desirable. For example, wireless communication techniques reduce, or eliminate, the wiring which is needed to connect to field devices. In such wireless configuration, there may be no process control loop available to power the field device. Therefore, an alternate power source must be used. For example, in some configurations, the field device is powered using stored power, for example, from a battery, obtained through solar cells, or other techniques. In such configurations, it is desirable to reduce the power consumption required by the field device. In other instances, the device is connected to a wired process control loop but further includes a wireless transceiver for sending or receiving communications. In such configurations, it is useful to conserve power in the device.
Also, it has become known to use a wireless adaptor to power a field device. Such wireless adaptors are described e.g. in published international patent applications WO 2010/018097 A1, US 2005/0245291 A1 and published international patent application WO 2008/150357 A1 which are hereby incorporated by reference.
Field device and wireless adaptor respectively nowadays comprise communication interfaces for communicating with other devices using e.g. one of the above mentioned communication protocols. As mentioned in published international patent application WO 2008/150357 A1 a wake-up circuit can be employed to wake up, i.e. fully power, such a communication interface in case it previously has been in a sleep-mode for conserving energy. For waking up such a communication interface of a field device, or an adaptor a wake-up signal can be employed. This wake-up signal is transmitted to the communication interface from an external device.
Additionally the need has arisen to manually power-up a field device on site e.g. when a measurement value is required and to conserve power, i.e. send the field device in a sleep mode or other power conservation mode when no measurement value is required.